📊 Percent Composition Calculator

Enter a chemical formula to get the mass percentage of each element.

Chemical Formula

—g/mol

Molar Mass

How to Use This Calculator

Type a chemical formula in the input box and click Calculate. The tool parses the formula, looks up the standard atomic mass for each element, and calculates what percentage of the total molar mass each element contributes. It supports parentheses for groups like Ca(OH)₂ and Al₂(SO₄)₃.

Type the chemical formula using standard notation: capital letter starts each element symbol, lowercase for the second letter, then the subscript number. Examples: H2O, CaCO3, C6H12O6, Ca(OH)2.

For compounds with polyatomic groups in parentheses, include them directly: Ca(OH)2 expands to Ca + O₂ + H₂. The parser handles nested parentheses too.

Click Calculate. The molar mass appears at the top, followed by each element with its mass percentage and its contribution in g/mol.

Verify the percentages sum to 100% (any small difference is rounding from atomic masses). If you get an error, check that element symbols are spelled correctly and capitalised properly.

The Percent Composition Formula

% element = (subscript count × atomic mass / molar mass) × 100% Molar mass = sum of (subscript × atomic mass) for all elements

The subscript count is how many atoms of that element appear in one formula unit. For H₂O, hydrogen has a subscript of 2. Multiply by the atomic mass (2 × 1.008 = 2.016 g/mol), divide by the molar mass (18.015 g/mol), and multiply by 100 to get 11.19% H.

Worked Examples

Water H₂O (M = 18.015 g/mol)H: 11.19%, O: 88.81%

Glucose C₆H₁₂O₆ (M = 180.16 g/mol)C: 40.00%, H: 6.71%, O: 53.29%

Calcium carbonate CaCO₃ (M = 100.09 g/mol)Ca: 40.05%, C: 12.00%, O: 47.95%

Sulfuric acid H₂SO₄ (M = 98.08 g/mol)H: 2.06%, S: 32.69%, O: 65.25%

Where This Calculation Comes Up

Percent composition is the starting point for any analysis that identifies compounds from their elemental makeup. When you run an elemental analyser (EA) in an organic chemistry lab, the instrument burns your sample and reports the mass percentages of C, H, N, and sometimes S. You compare those percentages to the theoretical values calculated from your proposed molecular formula to confirm the structure. If your synthesised compound gives 40.0% C, 6.7% H, and 53.3% O in the EA, it matches glucose (or any compound with empirical formula CH₂O) exactly.

Percent composition also comes up in industrial and environmental contexts. The nitrogen content of a fertiliser is reported as a percentage by mass. Urea CO(NH₂)₂ has a molar mass of 60.06 g/mol with 2 nitrogen atoms contributing 2 × 14.007 = 28.014 g/mol, giving 46.65% N. That is why urea is such a popular nitrogen fertiliser: nearly half its mass is nitrogen. Ammonium nitrate NH₄NO₃ contains 35.00% N. Knowing these percentages helps farmers and agronomists calculate how much fertiliser to apply per hectare to meet crop requirements.

Frequently Asked Questions

What is percent composition?

Percent composition is the percentage by mass of each element in a compound. It equals (mass of element / molar mass of compound) × 100%.

Why is percent composition useful?

It helps identify compounds, verify purity, determine empirical formulas from lab data, and is used extensively in analytical chemistry.

Example: percent composition of water (H₂O)?

H: (2×1.008)/18.015×100 = 11.19%. O: 15.999/18.015×100 = 88.81%.

Do percentages always add up to 100?

Yes, within rounding. If they don't sum to exactly 100%, it's due to rounding of atomic masses.

How is this related to empirical formula determination?

Percent composition data from lab analysis is the starting point for calculating empirical formulas. Each % is converted to moles.